The present invention generally relates to a magnetometer and more specifically relates to an electronic compass for use in a vehicle.
Magnetometers are used in many different applications. One such application is an electronic compass for a vehicle. In such electronic compasses, magnetometers are utilized to ascertain the vehicle heading relative to the Earth's magnetic north pole. A typical electronic compass includes two magnetic field sensors both disposed with their axes lying in a horizontal plane with a first sensor having its axis aligned in parallel with the longitudinal axis of the vehicle and the second sensor having its axis disposed orthogonal to the axis of the first sensor. The sensors are then utilized to detect the magnitude of orthogonal, horizontal, axially aligned components of the Earth's magnetic field vector such that a processing circuit may then compute the heading of the vehicle relative to the Earth's magnetic field vector.
Several different forms of magnetometers have been utilized for use in vehicle electronic compasses. Examples of some of these types of magnetometers include those utilizing flux-gate sensors, magneto-resistive sensors, and magneto-inductive sensors. Magneto-inductive sensors may be configured in different forms including L/R sensors and LC sensors. In both these forms of magneto-inductive sensors, a coil is wound around a core material. The sensor has a characteristic that its inductance varies linearly in response to a magnetic field, but only throughout two predetermined ranges of values of the external magnetic field. By viewing a plot of the sensor inductance versus the magnetic field strength (see FIG. 5, for example), one can see that the resultant curve is substantially symmetric about the point at which the magnetic field strength is zero. Accordingly, it is commonplace to apply a bias current to the sensor coil such that an artificial magnetic field is generated about the core material. The artificially generated magnetic field produced by this bias current is summed with the external magnetic field. External magnetic fields that are in the same direction as the artificial magnetic field generated by the bias current add to one another while an external magnetic field in the opposite direction of the artificial magnetic field is subtracted from the artificial magnetic field. Thus, by measuring the change in inductance of the sensor, the strength of the axially aligned magnetic field component may be ascertained.
To measure the inductance change of the sensor, circuit configurations where the responding frequency changes with changing sensor inductances have been employed. With such circuits, the changes in inductance of the sensor produces approximately proportional changes in the frequency of the signal output from the sensor. The frequency change may then be measured to determine the strength of the external magnetic field.
A problem encountered in such magnetometers is that the core material characteristics vary with temperature and age. One solution to this problem is disclosed in European Patent No. 0045509 B1. This European patent discloses that the bias current polarity on the sensor coil may be reversed with measurements taken with the bias current at both polarities such that the difference between the two measurements corresponds to the external magnetic field. The measurement thus taken is independent of any variance of the core material caused by temperature variation or age.
U.S. Pat. No. 5,239,264 discloses a similar technique. FIGS. 1 and 2 of this application correspond to FIGS. 3 and 4 of the '264 patent. As shown in FIGS. 1 and 2, the permeability function u(H) of the core material varies as a function of the strength of the magnetic field H over a particular range of the magnetic field strength. As apparent from this graph, there are generally two regions of the curve in which the permeability varies with respect to the change in magnetic field strength. One of these regions has a positive slope whereas the other region has a negative slope. In the '264 patent, the polarity of the DC bias current is alternatingly reversed so as to provide readings at both polarities. The two readings may then be subtracted from one another to arrive at the magnetic field strength of the component of the Earth's magnetic field sensed by that particular sensor coil.
In both the above-mentioned U.S. Pat. No. 5,239,264 and published European Patent No. 0045509 B1, the DC bias current remains at a constant level and only the polarity of the bias current is reversed. One problem with providing an electronic compass in an automobile is that the automobile may distort the external magnetic field. Further, as the vehicle travels past objects such as bridges, subways, power lines, railroad tracks, and other objects, these objects may cause disturbances in the magnetic field that are sensed by the electronic compass. Such magnetic field disturbances may produce magnetic fields that cause the magnetic field sensed by the sensor coils to fall within a non-linear region of the inductance versus magnetic field strength curve. Thus, the magnetometers of the above-described patents have limited ranges in which they can accurately detect the strength of the external magnetic field.
Accordingly, there exists a need for an electronic compass having the ability to accurately sense magnetic field components throughout a greater range than is presently provided by conventional magnetometers.